Valves are used to adjust the fluid flow through a system. One particular system where valves are used to control fluid flow are in heating, ventilating and air-conditioning systems (HVAC systems). For instance, liquid valves may be used to regulate water flowing through a heating system or valves in the form of dampers may be used to regulate cooled or heated air into an environment that is being conditioned.
At present, most HVAC systems have HVAC control systems that include end control devices, such as valve actuators, that control mechanical adjustment of end control elements, such as valves, in response to a control signal from an HVAC controller or building management system (BMS). Typically, a control signal is sent to the valve actuator and the valve actuator adjusts its output to change the position of the valve member of the valve (or damper) between a closed or open position in an open-close, floating, or modulating control manner. It is assumed that these changes in position of the end control element will result in a change in energy delivered to a controlled zone (via chilled or hot water heat transfer or conditioned air).
Unfortunately, valves do not have the same flow response curve from one type or size of valve to another type or size of valve. The same applies to dampers. Therefore, not all valves provide exactly the same flow vs. controlled valve position. Many HVAC control systems base their control signal that is sent to the valve actuator as if the valve has a flow profile that is an equal percentage flow curve. Unfortunately, because many, if not most, valves do not have a same flow profile and particularly not a flow profile that follows the equal percentage flow curve, tremendous amounts of error in controlling the flow of the valve exists.
For instance, for an equal percentage flow curve, the control system will typically expect about a 15% valve flow as a percentage of valve rated flow (Cv or Kv) when the valve is at a 50% valve position (i.e. half way between open and closed). In some families of valves (i.e. same style valve just change in valve size), the actual valve flow as a percentage of valve rated flow at the 50% valve position can range between 6% and 60% depending on the valve size. This variation from valve to valve can provide a significant error in the system considering the high accuracy of the command to the valve actuator from the HVAC controller.
These inherent errors from the theoretical flow curve (typically the equal percentage flow curve) can often cause system designers to oversize valves to insure that the system can provide enough flow rate under all conditions. This is because an undersized valve can never provide enough flow, and consequently carry enough energy, to meet all application needs. This habitual over sizing of the valves tends to cause the need for larger pumps, requiring more energy to supply the heating/cooling needs of the facility. Larger valves also tend to require larger heater radiator coils, raising the costs to install the actual HVAC system.
A further problem relating to over sized valves or significant error between the theoretical flow profile used by the HVAC control system and the actual flow profile of the valve is that the HVAC control system may cause significant overshoot in the control of the HVAC system such that the system cycles back and forth between high levels of heating followed by high levels of cooling to provide the desired conditioning of a zone. While the high level HVAC control systems may have sufficient feedback control to properly heat or cool the zone or environment, the system will be continuously fighting against itself causing inefficient operation of the HVAC system.